I changed all 4 Oxygen Sensors, two Catalytic Converters, and then reset the check engine light with diagnostic tool. But , the check engine light gets illuminated just after 25 miles of driving. I was the codes of 403 and 402 which says the Catalytic Converters below threshold.

I was reading that before and I still read that after installing to new Cats.

What seems to be the problem I can not figure it out. And it is not the Gas Cap either.

Do I need to reprogram the engine computer to fix this problem? Please help

It might not be but did you replace the battery before or after you replaced the cats? You asked about reprogramming the "engine computer" and I told you how to reset/reboot the engine control unit. Or are we talking about 2 different things?

Another possibility: Are you sure the new cats are compatible with your car and are installed properly?

This is an interesting one. The cats doesn't have any sensors and analysis of the cat operation depends on readings from the two o2 sensors before and after the first cat. Since you had installed MB cats, I think we can assume that they are operating correctly.
Now what may have happened if you had driven for a long time with bad cats and/or O2 sensors, is that the adaptive values for fuel trim, etc., may have tried to compensate for the bad parts and totally screwed up the mixture. Now that you have working parts in, the mixture may be totally wrong! If that is the case, some more driving cycles may bring the values into the correct range. Or you can also have all adaptations reset to the default values using STAR or Carsoft.
Of course you may also have a MAF going bad screwing up the fuel mixture, but that should normally throw codes as well. Are you sure you have the correct MAF fitted for the car?

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